NASA artist's vision of the moonsGanymede Europa Callisto near Jupiter

NASA cancelled the Jupiter Icy Moons Orbiter (JIMO) – an ambitious proposal to fly a spacecraft out to Jupiter, orbit three of its planet-sized moons, and study the vast saltwater oceans locked under their icy surfaces.

The moons to be orbited by JIMO would have been Europa, Callisto and Ganymede.

JIMO was to be a large-scale spacecraft carrying yet-to-be-developed technology and driven by electric propulsion powered by a nuclear fission reactor. The mission would have been launched sometime around 2015.

JIMO then would have visit Callisto first, then Ganymede, and then Europa.

What might have been: subsurface oceans?

NASA's interplanetary probe Galileo spotted evidence that the large icy moons have three ingredients essential for life – water, energy and necessary chemicals.

For instance, cracks, spots and pits on Europa's surface seem to be signs of an ocean of melted water under the moon's icy shell. Galileo data suggested the melted water had been in contact with the surface in geologically recent times and may remain relatively close to the surface.

The Galileo evidence for subsurface oceans has been ranked among the major scientific discoveries of the Space Age. JIMO would orbit each of the moons for extensive further investigation of their makeup, their history and their potential for sustaining life.

Why JIMO? The Jupiter Icy Moons Orbiter mission has two objectives:

To further human exploration by investigating the composition, history and potential for life on the prominent icy moons of Jupiter – Callisto, Ganymede, and Europa. Back in 2002, the U.S. National Research Council ranked the orbiting of Europa by an American probe as a top priority for a "flagship" Solar System mission. Adding Callisto and Ganymede would allow comparisons for understanding how those distant moons evolved.

To develop a nuclear reactor that can be processed safely and operated reliably in deep space for long-duration deep space exploration. The amount of power available from a nuclear reactor would be hundreds of times greater than other systems used on past interplanetary spacecraft. That would permit the use of more capable instruments and faster data transmission.

Calisto

Ganymede

Europa

Click image to learn more about the moon

Importance of water. If liquid water were to exist on the three icy moons, it would be reasonable to speculate on the existence of life there.

Could it have formed near undersea volcanic vents? After all, life here on Earth has been discovered at great ocean depths, beyond the penetration of sunlight, thriving near undersea volcanic vents on chemical nutrients upwelling from the interior of our planet.

On Earth, upwelling is the process in which nutrient-laden waters from the ocean depths rise toward the surface.

The Electric Ion Propulsion System

What could power JIMO to Jupiter? Its designers plan to use the electric drive system known as ion propulsion.

That kind of drive previously was used successfully by NASA's Deep Space 1 mission, which demonstrated it for interplanetary travel.

Deep Space 1 was launched in 1998. It flew by Asteroid 9969 Braille in1999 and then by Comet Borrelly in 2001. MORE ABOUT DEEP SPACE 1

The difference is Deep Space 1 drew electricity for its thrusters from solar panels. JIMO would draw its electricity from a nuclear fission reactor.

Reaching farther. JIMO would be a much more heavily instrumented craft than Deep Space 1 and would travel much farther from the Sun.

In fact, JIMO will increase dramatically NASAšs capability for space exploration by developing the use of electric propulsion powered by a nuclear fission reactor.

That technology will make it possible to consider such a large-scale mission as orbiting the moons of Jupiter one after the other. It also will open all of the vast unexplored regions of the outer Solar System for exploration.

Heat from JIMO's nuclear fission reactor would be converted to electricity, giving the probe more than 100 times as much power as a non-fission system of comparable weight.

Project Prometheus. NASAšs Project Prometheus will develop technology and conduct advanced studies in the areas of radioisotope power system and nuclear power and propulsion for the peaceful exploration of the Solar System.

Organized within the NASA Office of Space Science, Project Prometheus has the goal of developing America's first reactor power for a spacecraft and demonstrating that it can be operated safely and reliably on long duration missions in deep space.

determining the moons' interior structures, surface features and surface compositions to interpret their evolutionary histories.

knowing how geology, geochemistry and geophysics work on the moons will help us understand the origin and evolution of Earth.

The way the moons are weathered by material hitting their surfaces and the radiation environments around the moons:

the moons orbit within the powerful magnetic environment that surrounds the planet Jupiter.

they are effected by the natural radiation, charged particles and dust within that environment.

understanding the radiation and magnetic environment will hep us understand whether or not life arose on those distant moons.

What instruments will it carry? NASA will choose JIMO's science equipment once the project receives final approval. The space agency will ask scientists around the world to propose instruments to be carried aboard the Jupiter Icy Moons Orbiter.

JIMO surely will carry a radar for mapping the thickness of surface ice and a laser for mapping surface elevations.

Undoubtedly, it will have a camera, an infrared imager, a magnetometer, and instruments to study charged particles, atoms and dust that the spacecraft encounters near each moon.

The generous electrical power supplied by the nuclear system will be able to run instruments with higher power than those flown on other spacecraft and it will boost the data-transmission rate back to Earth.

The expanded scientific capacities should allow mapping the surfaces of Callisto, Ganymede and Europa with sufficient detail to see features as smaller than a house.

Special attention will be paid to identifying potential future landing sites.

The Flight Plan

After development and ground-testing of the rocket and spacecraft, a heavy lift expendable launch vehicle would lift the spacecraft into high Earth orbit sometime between 2012 and 2015.

JIMO then would separate from the rocket and the ion-propulsion thrusters would spiral the spacecraft away from Earth on its long run to Jupiter.

Years later, after entering orbit around Jupiter, JIMO would fly into individual orbits around Callisto, then Ganymede, and finally Europa.

Even considering the advanced radiation-resistant electronics that will be built into JIMO, the intensity of the radiation belts at Europa probably would limit how long the spacecraft's instruments would be able to operate in orbit around that moon. Thus, JIMO's work probably would end at Europa.